Recently, portable electric appliances such as camera-combined video tape recorders, portable telephones, lap-top computers and others have been being much popularized. From the viewpoint of environmental protection, the development of electric cars not discharging exhaust gas of NOx and the like has been being a subject matter for social discussion. Given that situation, studies for developing portable electric powers, and also clean energy sources of batteries, especially secondary batteries are being made actively. Above all, lithium or lithium ion secondary batteries are considered greatly hopeful, as producing higher energy density as compared with conventional aqueous electrolyte secondary batteries comprising lead cells or nickel-cadmium cells.
As the electrolytic solution for the lithium or lithium ion cell, widely used is a liquid comprising, as the electrolyte, a lithium-based electrolyte salt such as LiPF.sub.6 or the like, as dissolved in a non-aqueous, carbonate-based solvent such as low-molecular ethylene carbonate, propylene carbonate, diethyl carbonate or the like, since its electric conductivity is relatively high and since it is stable for cell potential.
Though having high capabilities, the non-aqueous electrolyte cell noted above is problematic in its safety, since a combustible organic solvent is used in the electrolytic solution therein. For example, one problem is that, when the cell is short-circuited, a large current rapidly passes through it to generate heat, thereby resulting in that the electrolytic solution containing an organic solvent in the cell is vaporized and decomposed to produce gas. Owing to the gas generation, the cell will be broken, exploded or ignited. One conventional method for solving this problem comprises equipping a safety valve or a current-blocking device capable of opening or acting depending on the increase in the inner pressure of the cell.
However, the improvement in the constitutional mechanism could not always apply to any and every problem with the cell. Therefore, it is necessary to drastically improve the cell material for the purpose of improving the safety capabilities of the cell.
The present invention has been proposed for the purpose of solving the problem noted above, and one object of the invention is to provide a non-aqueous electrolytic solution having excellent chemical and thermochemical stability. Another object of the invention is to provide a non-aqueous electrolyte cell having excellent cell capabilities, in which the electrolytic solution is prevented from being vaporized and decomposed thereby reducing the danger of cell breakage and ignition that may be caused by gas generation in the cell.
In order to attain the objects noted above, we, the present inventors have assiduously studied, and, as a result, have found that, when an inorganic polymer siloxane derivative which has high chemical stability and is hardly combustible or has a low vapor pressure is used as an electrolyte solvent in a cell, then the electrolytic solution comprising the solvent is prevented from being vaporized and decomposed in the cell, thereby reducing the danger of cell breakage and ignition, and that the cell comprising the electrolytic solution has excellent cell capabilities.